As electronic apparatuses and devices have higher performance and smaller size and weight, their materials are required to have higher performance and smaller particle size. For instance, magnetic particles for magnetic tapes are required to have smaller size and improved magnetization to enhance the magnetic recording density.
Also, to separate and collect proteins such as antigens, etc. for the diagnosis of sickness such as allergy, etc., magnetic separation methods have become widely used. As a result, increasingly higher demand is mounting to provide fine magnetic beads having high magnetization and excellent corrosion resistance.
Fine magnetic particles are mainly produced by liquid-phase synthesis methods such as a coprecipitation method, a hydrothermal synthesis method, etc. Fine magnetic particles obtained by the liquid-phase synthesis methods are oxide particles such as ferrite, magnetite, etc. A method of utilizing the thermal decomposition of an organometallic compound has recently become used; and fine magnetic Fe particles are formed, for instance, from Fe(CO)6.
Because magnetic metal particles have larger magnetization than that of oxide particles such as ferrite, their use for industrial applications is greatly expected. For instance, because the metal Fe has saturation magnetization of 218 Am2/kg, much larger than that of iron oxide, it is advantageous in providing excellent magnetic field response and large signal intensity. However, fine metal Fe particles are easily oxidized. When the fine metal Fe particles have a particle size of 100 μm or less, particularly 1 μm or less, they are vigorously burned in the air because of the increased specific surface area, resulting in difficulty in handling in a dry state. Accordingly, oxide particles such as ferrite, magnetite, etc. are widely used.
When the dried fine metal particles are handled, it is necessary to coat the particles lest that the metal is exposed to the air (oxygen). However, the metal would be oxidized considerably even by the method of JP 2000-30920 A, by which the particles are coated with an oxide of the particle-forming metal.
JP 9-143502 A proposes a method for producing graphite-coated, fine metal particles by mixing carbonaceous particles such as carbon black, natural graphite, etc. with particles of a metal or its compound (selected from metal oxides, metal carbides and metal salts), heat-treating the resultant mixture at 1600-2800° C. in an inert gas atmosphere, and cooling it at a speed of 45° C./minute or less. However, because metal-containing particles are heat-treated at an extremely high temperature of 1600-2800° C., this method may suffer the sintering of fine metal particles, and the production efficiency is low. Also, because graphite has a structure in which graphene sheets are laminated, its coating on spherical, fine metal particles inevitably has lattice defects. Accordingly, it is unsatisfactory for applications needing high corrosion resistance, such as magnetic beads, etc. Thus desired are fine metal particles having high corrosion resistance, and a low-cost method for producing such fine metal particles with excellent industrial productivity.